\documentclass{standalone}
\usepackage{tikz}
\usepackage{ctex,siunitx}
\setCJKmainfont{Noto Serif CJK SC}
\usepackage{tkz-euclide}
\usepackage{amsmath}
\usepackage{wasysym}
\usepackage{circuitikz}
\usetikzlibrary{patterns, calc}
\usetikzlibrary {decorations.pathmorphing, decorations.pathreplacing, decorations.shapes}
\ctikzset{bipoles/length=1cm}
\begin{document}
\small
\begin{circuitikz}[>=latex, scale=0.8,european]
  % \useasboundingbox(0.9,0)rectangle(5.1,5);
  \draw (0,0)--(3,0);
  \draw (0,4)--(1.5,4) to [R=$R_2$] (1.5,2) to [R=$R_1$, *-*] (1.5, 0);
  \draw (1.5,2)--(3,2);
  \draw [<->](3,0.1)--(3,1.9)node [midway,right] {$U_1$} ;
  \draw [<->](0,0.1)--(0,3.9)node [midway,right] {$U$} ;
  \draw [fill=white](0,0) circle (2pt);
  \draw [fill=white](0,4) circle (2pt);
  \draw [fill=white](3,0) circle (2pt);
  \draw [fill=white](3,2) circle (2pt); 
\end{circuitikz}
\end{document}